X-ray is a form of electromagnetic radiation that typically has a wavelength ranging from 0.01 to 10 nanometers (corresponding to frequencies in the range of 3×1016 Hz to 3×1019 Hz and photon energies in the range of 100 eV to 100 keV.
X-ray has been extensively used in imaging for about a century. Due to its penetrating ability, X-ray is useful to obtain images of the inside of subjects, e.g., in medical radiography and airport security. In X-ray imaging, an X-ray source generates X-ray; the X-ray is directed to a subject; the X-ray that is backscattered from or penetrates the subject forms a scene; an image sensor forms an image of the scene. One example of X-ray imaging techniques is fluoroscopy. Fluoroscopy is an imaging technique that uses X-rays to obtain real-time (or nearly real-time) moving images of the interior of a subject. In its primary application of medical imaging, a fluoroscope allows a physician to see the internal structure and function of a patient (e.g., the pumping action of the heart or the motion of swallowing). This is useful for both diagnosis and therapy and occurs in general radiology, interventional radiology, and image-guided surgery. In its simplest form, a fluoroscope consists of an X-ray source and a fluorescent screen, between which a patient is placed. Since the 1950s most fluoroscopes have included X-ray image intensifiers and cameras as well, to improve the image's visibility and make it available on a remote display screen.
Images obtained in X-ray imaging are usually 2D representations of the internal structure of the subject. Although techniques such as X-ray computed tomography (X-ray CT) is available to generate a 3D representation of the subject, these techniques often involve extensive computation from a large number of images, and thus are not suitable for real-time imaging.